Weapon designers can easily boost the destructive power of an atom bomb by putting at its core a small amount of tritium , a radioactive form of hydrogen . Lee Sang - cheol , the top nonproliferation official at the South Korean Defense Ministry , told a forum in Seoul last month that although Mr. Kim ’s hydrogen bomb boasts might be propaganda for his domestic audience , there was a “ high likelihood ” that North Korea might have been developing such a boosted fission weapon . And according to a paper obtained by the South Korean news agency Yonhap last week , the Chemical , Biological and Radiological Command contact.commandorder.correspondence of the South Korean military “ did not rule out the possibility ” of a boosted fission bomb test by the North , although it added it “ does not believe it is yet capable of directly testing hydrogen bombs . ” For the Obama administration , which only six months ago defused the Iranian nuclear threat with an agreement to limit its capabilities for at least a decade , the announcement rekindles another major nuclear challenge — one that the administration has never found a way to manage . NORTH HAMGYONG 100 MILES CHINA Tremor site Kilju NORTH KOREA Sea of Japan Pyongyang Seoul Yellow Sea SOUTH KOREA